Vanilla JS queue

queue.js

export default function Queue(initialValue = []) {
  // initialise the queue and offset
  var queue = initialValue
  var offset = 0

  // Returns the the queue.
  this.get = function() {
    return queue
  }

  // Returns the length of the queue.
  this.getLength = function() {
    return (queue.length - offset)
  }

  // Returns true if the queue is empty, and false otherwise.
  this.isEmpty = function() {
    return (queue.length === 0)
  }

  /* Enqueues the specified item. The parameter is:
   *
   * item - the item to enqueue
   */
  this.enqueue = function(item) {
    queue.push(item)
  }

  /* Dequeues an item and returns it. If the queue is empty, the value
   * 'undefined' is returned.
   */
  this.dequeue = function() {
    // if the queue is empty, return immediately
    if (!queue.length) return undefined

    // store the item at the front of the queue
    var item = queue[offset]

    // increment the offset and remove the free space if necessary
    if (++offset * 2 >= queue.length) {
      queue = queue.slice(offset)
      offset = 0
    }

    // return the dequeued item
    return item
  }

  /* Returns the item at the front of the queue (without dequeuing it). If the
   * queue is empty then undefined is returned.
   */
  this.peek = function() {
    return (queue.length > 0 ? queue[offset] : undefined)
  }
}

usage.js

import Queue from './queue'

const seed = localStorage.getItem(PERSISTANCE) || []

const queue = new Queue(seed)

const firstItemFromQueue = queue.dequeue()

// do stuff with firstItemFromQueue